window.onload = function () {
        var spread1 = document.getElementById("<%=FpSpread1.ClientID %>");
        if (document.all) {
            // IE
            if (spread1.addEventListener) {
                // IE9
                spread1.addEventListener("ContextMenuOpening", mopening, false);
            } else {
                // Other versions of IE and IE9 quirks mode (no doctype set)
                spread1.onContextMenuOpening = mopening;
            }
        } else {
            // Firefox
            spread1.addEventListener("ContextMenuOpening", mopening, false);
        }
    }

function mopening(event) {       
            alert("opening");      
    }